The invention relates to a dynamoelectric machine including permanent magnets which produce a magnetic field. More particularly, it relates to an annular assembly of a plurality of arcuate permanent magnets disposed in juxtaposition which may be used in such a machine.
Ceramic permanent magnets such as those formed of ferrite can be utilized in a dynamoelectric machine. It is recognized that a ceramic magnet can be magnetized in a radial direction, and has pole faces on itself. A high reluctance is presented along a path extending parallel to the pole faces, so that a need can advantageously be avoided for the provision of angularly spaced pole shoes or pole pieces which are required when ordinary Alnico magnets are used. Because of the difficulty to form a single ring of ceramic magnet which is radially magnetized, it is necessary to utilize a plurality of arcuate ceramic magnets disposed in an annular array in applications such as a dynamoelectric machine.
U.S. Pat. No. 3,663,850 issued to R. E. Phelon on May 16, 1972 discloses the use of an annular array of arcuate magnets in a dynamoelectric machine. In this instance, permanent magnets are initially arranged along the inside of a circular shipping band with a spacing between the ends of adjacent magnets into which elastic spacers or spring members are placed. These members act to exert a compressive pressure against the adjacent ends of the magnets, which are thus tightly held against the circular band to achieve a self-sustaining preassembly. The preassembly thus formed is applied against an annular internal surface of the machine which is of an equal diameter as the band, and the band removed, leaving the annular array of magnets held tightly against the machine surface.
The described arrangement requires the consecutive insertion of spacers or springs into the space between successive pairs of adjacent magnets, and it will be appreciated that the last one to be inserted must be placed by temporarily overcoming the pressures applied by oppositely located magnets and which are exerted by the remaining springs already held in place. To assure a positive self-sustaining effect, the spacers or springs must be of a relatively large size and must be of a material providing durability over a prolonged period. Hence, an increased spacing is required between the magnets, precluding the use of an array of increased number of closely spaced magnets. Since the described arrangement is adapted to be supported against its surrounding surface, the array cannot be mounted on a rotor or stator which is located inwardly thereof.